The present invention relates to a process of refining a C6-C16 aliphatic diol. More particularly, the process pertains to separating a purified C6-C16 aliphatic diol from a crude mixture containing the C6-C16 diol and one or more impurities selected from phenols, and aliphatic mono-ols, esters, carboxylic acids, hemiacetals, and mixtures thereof.
The aliphatic diols refined by the process of this invention find utility in the manufacture of plasticizers and as chain-extenders in the manufacture of polyurethanes.
As synthesized, C6-C16 aliphatic diols, preferably, C6-C16 alicyclic diols, such as, cis/trans 1,3- and 1,4-cyclohexanedimethanol, typically contain one or more impurities selected from the following classes of aliphatic compounds: mono-ols, esters, carboxylic acids, and hemiacetals, as well as impurity aromatic phenols, and mixtures of the aforementioned compounds. These impurities may be present in the starting materials from which the diol is prepared; or the impurities may be generated as side products during the process of preparing the diol. Diols derived from the hydrogenation of aldehydes, which themselves are generated by hydroformylation of olefins, are particularly susceptible to containing such impurities.
In a commercial example of the above, a cis, trans mixture of 1,3 and 1,4-cyclohexanedimethanol isomers (hereinafter referred to as “cis/trans-(1,3)(1,4)-cyclohexanedimethanol”) is typically prepared by first reacting 1,3-butadiene with acrolein in a Diels Alder reaction to form 3-cyclohexene-1-carboxaldehyde; and thereafter hydroformylating 3-cyclohexene-1-carboxaldehyde with a mixture of carbon monoxide and hydrogen (synthesis gas or syngas) to prepare cis/trans-(1,3)(1,4)-cyclohexane-dicarboxaldehyde, which is then hydrogenated in the presence of a hydrogenation catalyst to the crude aliphatic diol comprising cis/trans-(1,3)(1,4)-cyclohexanedimethanol and one or more of the aforementioned mono-ol, ester, carboxylic acid, hemiacetal, and/or phenol impurities. Preferably, the crude aliphatic diol is distilled to recover a purified cis/trans-(1,3)(1,4)-cyclohexanedimethanol product.
By analyzing various steps in the diol synthesis the skilled person may perceive the origins of the various impurities, although the invention described herein should not be bound to such theory. Hydroformylations are usually conducted in the presence of a rhodium-organophosphite ligand complex catalyst. As a side reaction, undesirable hydrolysis of the phosphite ligand may result in formation of impurity phenols. Impurity aliphatic mono-ols, such as cyclohexanemethanol and cyclohexenemethanol, may form as a side product of hydrogenation of the intermediate mono-aldehyde, for example, hydrogenation of cyclohexene-1-carboxaldehyde. Impurity carboxylic acids may be formed by oxidation of the aldehyde compositions or generated by hydrolysis of impurity esters. Impurity esters and hemiacetals may be derived from heavies that are present during distillation of crude aldehyde products.
Distillation of the crude C6-C16 aliphatic diol does not adequately remove the impurities, which may be sufficiently volatile to distill with the diol. Moreover, at a temperature sufficient for distillation, acidic impurities can catalyze formation of additional impurities through cracking of heavies, thereby increasing the overall quantity of impurities during the very distillation process that is intended to refine the diol. Acid impurities can also lead to corrosion of the distillation equipment. As a consequence, it would be desirable to discover a method of separating a purified C6-C16 aliphatic diol from a crude C6-C16 aliphatic diol contaminated with one or more impurities, particularly, those selected from aliphatic mono-ols, esters, carboxylic acids, and hemiacetals, as well as impurity aromatic phenols, and mixtures of the aforementioned compounds.
U.S. Pat. No. 6,632,331 discloses removal of aldehyde compounds contained as impurities in polycyclic diols by distilling the polycyclic diols in the presence of an alkali metal compound and/or an alkaline earth compound.
U.S. Pat. No. 6,117,277 discloses purification of C3-C10 straight-chain or branched mono-alcohols by distilling the alcohol at 150-200° C. in the presence of alkali metal hydroxide so as to remove impurity aldehydes.
U.S. Pat. No. 3,359,335 discloses purification of an alcohol product obtained by the catalytic carbonylation of an olefin with carbon monoxide and hydrogen to give a reaction mixture made up of carbonyl compounds, followed by catalytic hydrogenation to convert the carbonyl compounds to the alcohol product contaminated with high boiling esters, acetals, ethers, and decomposition products boiling within the boiling range of the alcohol. The purification involves scrubbing the crude alcohol product with an aqueous caustic solution, thereafter washing the scrubbed product to remove the caustic solution, and then distilling the washed product to recover a purified alcohol. Since polyols have appreciable solubility in water, significant product losses can result.